This invention relates to epoxy resin compositions having advantages including minimized formation of internal and surface voids, effective infiltration to semiconductor devices, and firm adhesion of cured compositions to substrates, and thus suitable for the encapsulation of semiconductor devices by potting, chip-on-bonding or screen printing.
Because of good cured physical properties, epoxy resin compositions are used in a wide variety of fields including electric and electronic fields. Upon application, they are given any suitable flow property, for example, an extremely low viscosity or a highly thixotropic state for ease of operation.
For the encapsulation of semiconductor devices, sealing methods including potting, chip-on-bonding (COB) and screen printing are now widely employed. The potting and COB methods require that the resin flow into very narrow gaps between gold wires extending from the semiconductor chip. The screen printing method requires that the resin maintain its shape after encapsulation. This means that a good compromise between thixotropy and flow must be found. Prior art liquid epoxy resin compositions suffer from the problem of short filling or infiltration.
Epoxy resin compositions also suffer from the problem of air bubbles resulting in defective parts. During potting or dripping, bubbles are often introduced, which cannot be removed during the curing step and are left as voids in the interior and on the surface. In the case of screen printing, bubbles are generated by a squeegee during printing and left as voids on the surface. In the case of transfer molding, air is carried in during flow from the gate to the cavity and left as internal voids.
It is well known to add surfactants to epoxy resin compositions in order to reduce voids. Known surfactants are fluorochemical surfactants such as Florade FC-170C, FC-430 and FC-431 from 3M-Sumitomo Co., Ltd. and silicone surfactants such as KF351, KF945 and KF618 from Shin-Etsu Chemical Co., Ltd. However, where silicone plasticizers are added to epoxy resin compositions, surfactants commonly used as anti-foaming agents are not effective for inhibiting the formation of bubbles. It is impossible to completely remove voids in such silicone component-containing compositions. A study has not been made on the anti-foaming problem associated with the sealing methods including potting, COB and screen printing. It is merely attempted to reduce the viscosity or thixotropy of the resin in order to improve infiltration.
An object of the invention is to provide epoxy resin compositions having advantages including minimized formation of internal and surface voids, effective infiltration to semiconductor devices, and firm adhesion of cured compositions to substrates.
We have found that in an epoxy resin composition comprising (A) an epoxy resin, (B) a curing agent and (C) a silicone stress-reducing agent, the addition of (D) a foam-suppressing composition comprising (D-i) an oil compound consisting of a hydrophobic organopolysiloxane of the general formula (1) shown below and finely divided silica, (D-ii) a hydrophilic polyoxyalkylene-modified silicone oil of the general formula (2) shown below, and optionally, (D-iii) a polyoxyalkylene polymer is effective for eliminating and breaking bubbles. As a consequence, the formation of voids within the interior or on the surface of a cured product is fully inhibited. The epoxy resin composition is given an ability to infiltrate into semiconductor devices. The composition is fully adherent to various substrates. The addition of foam-suppressing composition (D) does not affect the physical properties of the cured composition. Therefore, the resultant epoxy resin composition is advantageously employed in the encapsulation of semiconductor devices by such techniques as potting, COB and screen printing.
According to the invention, there is provided an epoxy resin composition comprising (A) an epoxy resin, (B) a curing agent, (C) a silicone stress-reducing agent, and (D) a foam-suppressing composition. The foam-suppressing composition (D) is comprised of:
(D-i) 5 to 60% by weight of an oil compound consisting of 100 parts by weight of a hydrophobic organopolysiloxane of the following general formula (1):
R1aSiO(4xe2x88x92a)/2xe2x80x83xe2x80x83(1) 
wherein R1 is at least one group selected from hydroxyl groups and substituted or unsubstituted monovalent hydrocarbon groups of 1 to 18 carbon atoms, and xe2x80x9caxe2x80x9d is a number of 1.9 to 2.2, having a viscosity of 10 to 500,000 centistokes at 25xc2x0 C., and 0.1 to 20 parts by weight of finely divided silica having a BET specific surface area of at least 100 m2/g,
(D-ii) 5 to 95% by weight of a hydrophilic polyoxyalkylene-modified silicone oil of the following general formula (2):
R4R22SiOxe2x80x94(R22SiO)x(R2R3SiO)yxe2x80x94SiR22R4xe2x80x83xe2x80x83(2) 
wherein R2 which may be the same or different is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 18 carbon atoms, R3 is a group of the formula: xe2x80x94R5O(R6O)b(R7O)C-R8 wherein R5 is a divalent C1-C4 hydrocarbon group, R6 is ethylene, R7 is propylene, R8 is hydrogen or a monovalent organic group selected from the class consisting of C1-C6 alkyl, acetyl and isocyanate groups, b is an integer of 3 to 100, c is an integer of 0 to 100, R4 is as defined for R2 or R3, x is an integer of 5 to 100, and y is an integer of 1 to 10, having a cloud point in 1% aqueous solution of at least 40xc2x0 C. and a viscosity of 10 to 100,000 centistokes at 25xc2x0 C., and
(D-iii) 0 to 90% by weight of a polyoxyalkylene polymer having a molecular weight of 500 to 5,000. The foam-suppressing composition (D) is present in an amount of 1xc3x9710xe2x88x924to 20 parts by weight per 100 parts by weight of components (A) and (B) combined.
Briefly stated, the epoxy resin composition of the invention is defined as comprising (A) an epoxy resin, (B) a curing agent, (C) a silicone stress-reducing agent, and (D) a foam-suppressing composition.
The epoxy resin (A) used herein is not particularly limited with respect to molecular structure, molecular weight and other factors as long as it has at least two epoxy groups per molecule. Examples of the epoxy resin which can be used herein include bisphenol type epoxy resins such as bisphenol A type epoxy resins and bisphenol F type epoxy resins; novolac type epoxy resins such as phenol novolac type epoxy resins and cresol novolac type epoxy resins; triphenolalkane type epoxy resins such as triphenolmethane type epoxy resins and triphenolpropane type epoxy resins; phenolaralkyl type epoxy resins, biphenylaralkyl type epoxy resins, stilbene type epoxy resins, naphthalene type epoxy resins, biphenyl type epoxy resins, and cyclopentadiene type epoxy resins. These epoxy resins may be used alone or in admixture of two or more.
Where liquid epoxy resin compositions are desirable, there may be used any epoxy resin which has at least two epoxy groups per molecule and is liquid at room temperature (typically 25xc2x0 C.). Preferred are bisphenol type epoxy resins such as bisphenol A type epoxy resins and bisphenol F type epoxy resins, and naphthalene type epoxy resins.
The curing agent (B) used herein is not particularly limited with respect to molecular structure, molecular weight and other factors. Any compound known as the curing agent may be used as long as it has at least two functional groups capable of reacting with epoxy groups in the epoxy resin. Exemplary functional groups are phenolic hydroxyl groups, amino groups and acid anhydride groups (with the proviso that the compound may have at least one acid anhydride group). A variety of phenolic resins are useful, including phenolic resins having at least two phenolic hydroxyl groups per molecule, for example, novolac type phenolic resins such as phenol novolac resins and cresol novolac resins; xylylene-modified novolac resins such as p-xylylene-modified novolac resins, m-xylylene-modified novolac resins, and o-xylylene-modified novolac resins; bisphenol type phenolic resins such as bisphenol A type resins and bisphenol F type resins; biphenyl type phenolic resins, resol type phenolic resins, phenol aralkyl type resins, biphenyl aralkyl type resins, triphenolalkane type resins and polymers thereof such as triphenolmethane type resins and triphenolpropane type resins; naphthalene ring-bearing phenolic resins; and dicyclopentadiene-modified phenolic resins. Amine and acid anhydride type curing agents are also useful.
Of these, phenolic resins are preferable where the inventive composition is adapted for transfer molding.
Where liquid epoxy resin compositions are desirable, acid anhydrides are preferred curing agents. Exemplary acid anhydrides are those of about 4 to 25 carbon atoms, preferably about 8 to 20 carbon atoms, preferably containing one or two aliphatic or aromatic rings and one or two acid anhydride groups (i.e., xe2x80x94COxe2x80x94Oxe2x80x94COxe2x80x94) in a molecule, such as tetrahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, hexahydrophthalic anhydride, methylhymic anhydride, pyromellitic acid dianhydride, benzophenonetetracarboxylic acid dianhydride, 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic acid dianhydride, bis(3,4-dicarboxyphenyl)ether dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, and 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride.
As the curing agent for use in liquid epoxy resin compositions, dicyandiamide and carboxylic acid hydrazides such as adipic acid hydrazide and isophthalic acid hydrazide are also useful.
The amount of the curing agent blended is an effective amount to cure the epoxy resin and selected as appropriate from a wide range. Where a phenolic resin is used as the curing agent, it is used in such amounts as to give 0.5 to 2.0 mol, especially 0.8 to 1.3 mol of phenolic OH groups per mol of epoxy groups in the epoxy resin. Where an acid anhydride is used as the curing agent, it is preferably blended so as to give 0.3 to 0.7 mol of acid anhydride (xe2x80x94COxe2x80x94Oxe2x80x94COxe2x80x94) groups per mol of epoxy groups in the epoxy resin. Less than 0.3 mol of acid anhydride groups may induce insufficient cure whereas an amount giving more than 0.7 mol of acid anhydride groups may leave part of the acid anhydride unreacted, which leads to a drop of glass transition temperature. It is more desirable that 0.4 to 0.6 mol of acid anhydride groups be available per mol of epoxy groups in the epoxy resin.
If desired, a curing accelerator is used for accelerating curing reaction of epoxy resin (A) with curing agent (B). Illustratively, one or more curing accelerators selected from imidazole derivatives, tertiary amine compounds, and organic phosphorus compounds may be blended. Exemplary imidazole derivatives are 2-methylimidazole, 2-ethylimidazole, 4-methylimidazole, 4-ethylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 2-phenyl-4-hydroxymethylimidazole, 2-ethyl-4-methylimidazole, 1-cyanoethyl-2-methylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, and 2-phenyl-4,5-dihydroxy-methylimidazole.
Exemplary tertiary amine compounds are amine compounds having alkyl or aralkyl groups as the substituent attached to the nitrogen atom such as triethylamine, benzyltrimethylamine, and xcex1-methylbenzyldimethylamine; cycloamidine compounds and salts thereof with organic acids such as 1,8-diazabicyclo[5.4.0]undecene-7 and the phenol salt, octylic acid salt, and oleic acid salt thereof; salts or complex salts of cycloamidine compounds with quaternary boron compounds such as the compound represented by the following formula. 
Exemplary organic phosphorus compounds are triorgano-phosphine compounds such as triphenylphosphine, tributylphosphine, tri(p-methylphenyl)phosphine, tri(nonylphenyl)phosphine, and triphenylphosphine triphenylboran, and quaternary phosphonium salts such as tetraphenylphosphonium tetraphenyl borate.
An appropriate amount of the curing accelerator blended is 0.01 to 10 parts, and desirably 0.5 to 5 parts by weight per 100 parts by weight of the epoxy resin and curing agent combined. Less than 0.01 part of the curing accelerator may not be effective for accelerating cure whereas more than 10 parts is effective for accelerating cure, but tends to detract from storage stability.
The silicone stress-reducing agent (C) may be selected from among silicone rubbers, silicone oils, silicone-modified resins, and copolymers of silicone polymers with organic polymers such as substituted or unsubstituted phenol novolac resins and epoxy resins. Of the copolymers, preferred are block copolymers of alkenyl group-bearing epoxy resins or phenolic resins with SiH group-bearing organopolysiloxane, resulting from hydrosilylation addition reaction between alkenyl groups on the epoxy resins or phenolic resins and SiH groups on the organopolysiloxane. The blending of the silicone stress-reducing agent is effective for minimizing the generation of cracks in cured packages on a thermal shock test.
Preferably, the silicone polymers used to form the copolymers are of the following average compositional formula (4).
R12dR13eSiO(4xe2x88x92dxe2x88x92e)/2xe2x80x83xe2x80x83(4) 
Herein, R12 is hydrogen, an organic group having a functional group selected from among hydroxyl, amino, epoxy, carboxyl, alkoxy, alkoxyalkoxy and alkenyloxy groups, an alkoxy group, or an alkenyloxy group; R13 is a substituted or unsubstituted monovalent hydrocarbon group; d and e are numbers satisfying 0.001xe2x89xa6dxe2x89xa61, 1xe2x89xa6exe2x89xa63 and 1xe2x89xa6d+e less than 4. The number of silicon atoms per molecule is an integer of 2 to 1,000. The number of silicon atom-bonded R12 groups per molecule is an integer of at least 1.
Examples of R12 group include hydrogen, organic groups having a functional group selected from among hydroxyl, amino, epoxy, carboxyl, alkoxy groups (e.g., methoxy, ethoxy, n-propoxy, butoxy, methoxyethoxy and ethoxyethoxy), alkoxyalkoxy groups and alkenyloxy groups. R13 represents substituted or unsubstituted monovalent hydrocarbon groups, preferably of 1 to 10 carbon atoms, for example, alkyl groups such as methyl, ethyl, n-propyl and n-butyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl and tolyl; aralkyl groups such as benzyl and phenylethyl; and the foregoing groups in which some or all of the hydrogen atoms are substituted with halogen atoms, alkoxysilyl groups or the like, such as chloromethyl, 3,3,3-trifluoropropyl, trimethoxysilylethyl, and methyldimethoxysilylethyl.
The silicone polymer preferably has a molecular weight of 100 to 70,000, though it is not limited thereto. This is because an organopolysiloxane or silicone polymer having a molecular weight of 100 to 70,000 ensures that when a copolymer of an aromatic polymer with an organopolysiloxane or silicone polymer is blended in a thermosetting resin composition, the copolymer remains incompatible with the matrix and forms a fine island-in-sea structure. With a molecular weight of less than 100, the resulting copolymer may fail to impart a friction ability or a high glass transition temperature when blended in a thermosetting resin composition. If the molecular weight of organopolysiloxane exceeds 70,000, the resulting copolymer may become incompatible with the thermosetting resin and be separated, resulting in poor flexural strength.
For the purpose of stress reduction, the silicone stress-reducing agent (C) is preferably contained in an amount of 1 to 25 parts by weight per 100 parts by weight of the epoxy resin and the curing agent combined. The more preferred amount is 2 to 15 parts by weight. Less than 1 part of the stress-reducing agent may be ineffective for the purpose whereas blending of more than 25 parts of the stress-reducing agent may increase the viscosity of the epoxy resin composition to interfere with molding thereof.
While the epoxy resin composition is described above as comprising the epoxy resin (A), curing agent (B) and silicone stress-reducing agent (C), the present invention is characterized in that (D) a foam-suppressing composition comprising (D-i) an oil compound consisting of a hydrophobic organopolysiloxane of the general formula (1) shown below and finely divided silica, (D-ii) a hydrophilic polyoxyalkylene-modified silicone oil of the general formula (2) shown below, and optionally, (D-iii) a polyoxyalkylene polymer is blended in the composition in an amount of 1xc3x9710xe2x88x924 to 20 parts by weight per 100 parts by weight of components (A) and (B) combined.
Component (D-i) is an oil compound consisting of 100 parts by weight of a hydrophobic organopolysiloxane of the following general formula (1) having a viscosity of 10 to 500,000 centistokes at 25xc2x0 C. and 0.1 to 20 parts by weight of finely divided silica having a BET specific surface area of at least 100 m2/g.
R1aSiO(4xe2x88x92a)/2xe2x80x83xe2x80x83(1) 
Herein R1 is selected from hydroxyl groups and substituted or unsubstituted monovalent hydrocarbon groups of 1 to 18 carbon atoms, and xe2x80x9caxe2x80x9d is a positive number of 1.9 to 2.2. Illustratively, R1 represents hydroxyl groups or substituted or unsubstituted monovalent hydrocarbon groups of 1 to 18 carbon atoms, especially 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl and tolyl; aralkyl groups such as 2-phenylethyl and 2-methyl-2-phenylethyl; and the foregoing groups in which some or all of the hydrogen atoms attached to carbon atoms are substituted with halogen atoms, cyano groups, amino groups or the like, such as chloromethyl, 3-chloropropyl, 3,3,3-trifluoropropyl, cyanoethyl, 3-aminopropyl, and N-(xcex2-aminoethyl)-xcex3-aminopropyl. R1 groups may be the same or different although it is preferred that at least 80 mol % of the entire R1 groups be methyl.
The organopolysiloxane is merely required to be liquid at room temperature although the anti-foaming and working standpoints require it to have a viscosity of 10 to 500,000 centistokes at 25xc2x0 C., and preferably 1,000 to 100,000 centistokes at 25xc2x0 C.
The finely divided silica used in (D-i) may be well-known one and either wet silica or dry silica. Examples include precipitated silica, silica xerogel, and fumed silica, which may have been surface treated with organic silyl groups. Suitable silica is commercially available under the trade name of Aerosil by Nippon Aerosil Co., Ltd., Nipsil by Nippon Silica Co., Ltd., Cab-O-Sil by Cabot Corp. and Santocel by Monsanto Co.
From the standpoints of affinity to and dispersion in silicone oil, the finely divided silica should have a specific surface area of at least 100 m2/g, usually 100 to 500 m2/g, preferably 100 to 400 m2/g as measured by the BET method. The amount of silica added should be 0.1 to 20 parts, preferably 1 to 15 parts by weight per 100 parts by weight of the hydrophobic organopolysiloxane. Less than 0.1 part of silica is ineffective for anti-foaming whereas with more than 20 parts of silica, the composition becomes too viscous and difficult to disperse and work.
The silicone oil compound (D-i) can be obtained by admixing predetermined amounts of the hydrophobic organopolysiloxane and finely divided silica, heat treating the mixture at room temperature or a temperature below 200xc2x0 C., and optionally, removing a low-boiling fraction therefrom. If desired for the purposes of sustaining an anti-foaming ability and improving high-temperature properties and dilution stability, any of inorganic ammonium salts, organosilicon compounds and siloxane resins may be added (see JP-B 4-42043, JP-A 5-261206 and JP-A 5-261207).
In the foam-suppressing composition (D), the content of the oil compound (D-i) is 5 to 60%, preferably 10 to 50%, and more preferably 20 to 40% by weight. The composition (D) with less than 5% (D-i) is less effective for foam suppression whereas more than 60% (D-i) is less dispersible and awkward to handle.
Component (D-ii) in the foam-suppressing composition (D) is a hydrophilic polyoxyalkylene-modified silicone oil which helps the silicone oil compound (D-i) be emulsified and dispersed in the system, optionally by cooperating with the polyoxyalkylene polymer (D-iii). This silicone oil is represented by the following general formula (2).
R4R22SiO-(R22SiO)x-(R2R3SiO)y-SiR22R4xe2x80x83xe2x80x83(2) 
Herein R2 represents substituted or unsubstituted monovalent hydrocarbon groups of 1 to 18 carbon atoms, especially 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, and octadecyl; cycloalkyl groups such as cyclohexyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl and tolyl; aralkyl groups such as 2-phenylethyl and 2-methyl-2-phenylethyl; and the foregoing groups in which some or all of the hydrogen atoms attached to carbon atoms are substituted with halogen atoms, cyano groups, amino groups or the like, such as chloromethyl, 3-chloropropyl, 3,3,3-trifluoropropyl, cyanoethyl, 3-aminopropyl and N-(xcex2-aminoethyl)-xcex3-aminopropyl. The R2 groups may be the same or different although it is preferred that at least 90 mol % of the entire R2 groups be methyl.
R3 is a group of the formula: -R5O(R6O)b(R7O)c-R8. R5 is a divalent hydrocarbon group having 1 to 4 carbon atoms, for example, a C1-C4 alkylene group such as methylene, ethylene, propylene or butylene. R6 is ethylene, and R7 is propylene. R8 is hydrogen or a monovalent organic group selected from among alkyl groups having 1 to 6 carbon atoms (e.g., methyl, ethyl, propyl, butyl, pentyl and hexyl), acetyl groups, and isocyanate groups. The letter b is an integer of 3 to 100, preferably 10 to 50. If b is less than 3, it is difficult to emulsify the oil compound composition. If b is more than 100, the oil compound composition becomes viscous when diluted and awkward to process. The letter c is an integer of 0 to 100, preferably 0 to 20. If c is more than 100, it is difficult to emulsify the oil compound composition.
R4 is as defined for R2 or R3. 
The letter x is an integer of 5 to 100, and especially 10 to 60. If x is less than 5, the anti-foaming effect becomes poor and the storage stability is exacerbated by a drop of viscosity. If x is more than 100, the oil compound composition becomes viscous when diluted and awkward to process. The letter y is an integer of 1 to 10, and especially 3 to 7. If y is less than 1, it is difficult to emulsify the oil compound composition. If y is more than 10, the oil compound composition becomes viscous when diluted and awkward to process.
The polyoxyalkylene-modified silicone oil (D-ii) should have a cloud point of at least 40xc2x0 C., preferably at least 60xc2x0 C., as measured in a 1% aqueous solution thereof. If the cloud point is lower than 40xc2x0 C., the foam-suppressing composition may become unstable at high temperature.
Preferably the polyoxyalkylene-modified silicone oil is liquid at room temperature. It should have a viscosity of 10 to 100,000 centistokes at 25xc2x0 C., preferably 100 to 10,000 centistokes at 25xc2x0 C. If the viscosity is less than 10 cSt, the foam-suppressing composition may become separable and less stable. A viscosity in excess of 100,000 cSt may adversely affect the dispersion and processing of the foam-suppressing composition.
Commercially available ether-modified silicones may be used as the polyoxyalkylene-modified silicone oil. Alternatively, it is readily obtained by conventional well-known methods, for example, by effecting addition reaction of a polyoxyalkylene compound having aliphatic unsaturated groups (e.g., vinyl or allyl) at ends of its molecular chain to an organopolysiloxane having xe2x89xa1SiH groups in the presence of a platinum catalyst.
Included are not only silicone oil modified with a single polyoxyalkylene compound, but also silicone oil having incorporated within the same molecule two or more types of polyoxyalkylene groups having different molar ratio or molecular weight, for example, ethylenoxy and propylenoxy groups. Two or more polyoxyalkylene-modified silicone oils of different structure may also be used.
In the foam-suppressing composition (D), the content of the modified silicone oil (D-ii) is 5 to 95%, preferably 10 to 80%, and more preferably 20 to 70% by weight. Less than 5% of (D-ii) is ineffective for helping the anti-foaming agent be dispersed whereas more than 95% of (D-ii) adversely affects the anti-foaming function and rather allows for foaming.
Component (D-iii) in the foam-suppressing composition (D) is a polyoxyalkylene polymer which serves as an emulsifying aid to cooperate with component (D-ii). The blending of component (D-iii) lowers the viscosity of the foam-suppressing composition and thus improves the working efficiency during dispersion. The polyoxyalkylene polymer (D-iii) is not limited in structure as long as it has (R9O) groups in a molecule wherein R9 is an ethylene or propylene group. It may be linear or branched as represented by the following general formula (3). 
Herein, R10 is hydrogen or a monovalent organic group having 1 to 18 carbon atoms, especially 1 to 8 carbon atoms, such as an alkyl, aryl or alkenyl group, R11 is an alkyl group having 1 to 18 carbon atoms, q, r and s are integers.
The terminal groups represented by R10 generally include hydrogen, alkyl groups such as methyl, ethyl, butyl, decyl and octadecyl, aryl groups such as phenyl and tolyl, alkenyl groups such as vinyl and allyl, acyl groups such as acetyl and stearoyl, and isocyanate groups, though are not limited thereto.
The polyoxyalkylene polymer should have an average molecular weight of 500 to 5,000, preferably 1,000 to 3,000. A polyoxyalkylene polymer with a molecular weight of less than 500 has a low viscosity so that the resulting foam-suppressing composition may have poor aging stability. A polyoxyalkylene polymer with a molecular weight of more than 5,000 has a high viscosity and becomes difficult to disperse and process. Component (D-iii) is not limited to a single polyoxyalkylene polymer, and two or more polyoxyalkylene polymers having different HLB or different terminal groups may be used in combination.
In the foam-suppressing composition (D), the content of the polyoxyalkylene polymer (D-iii) is 0 to 90%, preferably 0 to 50%, and more preferably 0 to 40% by weight. More than 90% of (D-iii) adversely affects the anti-foaming function.
The foam-suppressing composition (D) may be obtained by mixing predetermined amounts of the above-described components (D-i), (D-ii) and (D-iii). It is only necessary to intimately mix the components using a mixer such as a homomixer. The mixing method is not critical. For example, the components may be mixed while heating at 40 to 150xc2x0 C.
The foam-suppressing composition (D) is formulated in the epoxy resin composition in an amount of 1xc3x9710xe2x88x924 to 20 parts, preferably 1xc3x9710xe2x88x922 to 10 parts by weight per 100 parts by weight of components (A) and (B) combined. Less than 1xc3x9710xe2x88x924 part of the foam-suppressing composition fails to achieve the desired foam-eliminating and breaking effects whereas more than 20 parts rather allows for foaming, adversely affects adhesion, and bleeds on the surface causing stains on the cured surface.
In the epoxy resin composition, any of well-known inorganic fillers may be added for the purpose of reducing the coefficient of expansion of cured composition. Useful inorganic fillers include fused silica, crystalline silica, alumina, boron nitride, aluminum nitride, silicon nitride, magnesia, and magnesium silicate. If semiconductor devices to be encapsulated generate a substantial amount of heat, it is desired to use inorganic fillers having a higher thermal conductivity and a lower coefficient of expansion, for example, alumina, boron nitride, aluminum nitride and silicon nitride, optionally in admixture with fused silica.
It is desired that the inorganic filler have such a particle size distribution that the mean particle size is 2 to 30 xcexcm and the maximum particle size is up to 74 xcexcm. A mean particle size of less than 2 xcexcm may lead to a higher viscosity, inhibiting loading of a large amount of filler. A mean particle size of more than 30 xcexcm means a more amount of coarse particles which can cause gate clogging and blocking of a thin needle in a dispenser. The mean particle size as used herein can be determined, for example, as the weight average (median diameter) in the particle size distribution as measured by the laser light diffraction method.
The shape of the inorganic filler is not critical. Inorganic fillers in flake, dendrite and spherical forms may be used alone or in admixture of any. If desired, ultrafine silica such as Aerosil is added for imparting thixotropy.
The inorganic filler is preferably blended in an amount of about 100 to 1,000 parts, more preferably about 200 to 900 parts by weight per 100 parts by weight of epoxy resin (A) and curing agent (B) combined.
It is preferred to previously surface treat the inorganic filler with coupling agents such as silane coupling agents and titanate coupling agents because low water absorption, impact resistance and crack resistance are further improved.
Preferred coupling agents are silane coupling agents including epoxy-functional alkoxysilanes such as xcex3-glycidoxypropyltrimethoxysilane, xcex3-glycidoxypropyl-methyldiethoxysilane, and xcex2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane, amino-functional alkoxysilanes such as N-xcex2-(aminoethyl)-xcex3-aminopropyltrimethoxysilane, xcex3-aminopropyltriethoxysilane, and N-phenyl-xcex3-aminopropyl-trimethoxysilane, and mercapto-functional alkoxysilanes such as xcex3-mercaptopropyltrimethoxysilane. The amount of coupling agent used and the surface treating method are not critical.
In the epoxy resin composition, any of well-known diluents such as n-butyl glycidyl ether, phenyl glycidyl ether, styrene oxide, t-butyl phenyl glycidyl ether, and dicyclopentadiene epoxide may be added for the purpose of lowering the viscosity.
Besides the above-described silane coupling agents and titanate coupling agents, there may be added other coupling agents such as aluminum coupling agents. Also if desired, there may be added colorants such as carbon black, and surfactants other than the foam-suppressing composition (D), such as nonionic surfactants and fluorochemical surfactants serving as a wetter or anti-foaming agent. These additives may be added in customary amounts insofar as the benefits of the invention are not impaired.
The inventive epoxy resin composition may be prepared by agitating, melting, mixing and dispersing the epoxy resin and the curing agent at the same time or separately, optionally under heating, and adding the silicone stress-reducing agent, the foam-suppressing composition and optional additives to the mixture, followed by mixing, agitation and dispersion. The means for use in the mixing, agitation and dispersion of the mixture is not critical although mixers having agitating and heating functions such as attritors, three-roll mills, ball mills and planetary mixers are often used. Such units may be appropriately combined.
Where the epoxy resin composition is used in transfer molding, it may be prepared by intimately mixing the epoxy resin, curing agent, silicone stress-reducing agent, foam-suppressing composition, and optional additives in a high-speed agitating device, then milling the resulting mixture using a hot two-roll mill, continuous kneader or the like. Such units may be appropriately combined.
The epoxy resin compositions of the invention can be readily cured simply by heating. The curing conditions are not critical although heating at about 100 to 190xc2x0 C. for about xc2xd to 10 hours is desirable.
The epoxy resin compositions of the invention have advantages including effective infiltration into gaps between semiconductor devices and substrates, few voids formed in the interior and on the surface, and firm adhesion of cured compositions to various substrates. The compositions are thus suitable for the encapsulation of semiconductor devices by potting, COB and screen printing techniques.